1. Field of the Invention
The present invention relates to a spark plug.
2. Description of the Related Art
A spark plug has a cylindrical metal shell, in which a cylindrical insulator extends in the axial direction of the metal shell and is fixed to the inner side of the metal shell. Moreover, a center electrode extends in the axial direction of the metal shell such that its leading end protrudes from the insulator and its trailing end is fixed to the insulator. On the other hand, one end of the ground electrode is fixed to the metal shell and its other end portion forms a discharge gap with the leading end of the center electrode.
The ground electrode of a typical spark plug includes an electrode body made of a Ni alloy such as INCONEL 600 (product of Special Metals Corp., USA) or the like so as to exhibit excellent durability, and a tip made of a material resistant to spark discharge such as a Pt—Ir alloy welded to a portion of the ground electrode to form the discharge gap side of the electrode body. A spark plug having a ground electrode made of a Pt—Ir alloy as a whole is also known. Moreover, the spark plug, as disclosed in JP-A-5-74549, has a ground electrode composed of a core portion and a coating portion enveloping the core portion. The core portion is made of Ni, Cu or an alloy thereof, and the coating portion is made of a Pt—Ir alloy.